The present invention relates generally to communications systems, and more particularly, to decoding convolutionally encoded messages such as paging messages transmitted within a code division multiple access (CDMA) wireless telecommunications system.
Currently, standard protocols for CDMA wireless telecommunications define requirements for various communication channels, including paging and access channels, traffic channels and synchronization channels. A paging channel is used by a base station to communicate to mobile stations (radiotelephones) when they are not already assigned to a traffic channel. The primary purpose of the paging channel is to convey pages, i.e., notifications of incoming calls, to the mobile stations.
The mobile station operates in an idle mode while awaiting notification of an incoming call over the paging channel. Typically, to conserve power in the idle mode and thereby prolong battery life, the mobile station operates in a slotted mode, in which the mobile station periodically activates its receiver electronics to xe2x80x9clistenxe2x80x9d for a page during a periodic time slot allocated to at least that mobile station. If no page intended for that mobile station is received during the allocated time slot, the mobile station xe2x80x9cgoes to sleepxe2x80x9d, essentially turning off its components and conserving power, until it reactivates itself for the next assigned slot. In general, the percentage of time that the mobile station is allowed to sleep in the idle mode is proportional to the power conservation.
In accordance with the TIA-IS-95-A standard, which governs CDMA wireless communications in the United States, paging messages are convolutionally encoded in the paging channel. This technique reduces the likelihood of errors upon reception of the message due to multipath fading, noise or other factors. Such convolutional encoding involves sequencing an original data stream through a shift register and summing the bits of predetermined stages of the shift register to generate a larger number of bits for transmission. Mobile stations are permitted to operate either in a slotted mode as mentioned above to receive synchronized pages, or in a non-slotted mode to receive unsynchronized or synchronized pages. The pages transmitted by the base station are of variable length, up to 80 ms in duration. At the mobile station, decoding of the page message is preferably performed with a Viterbi decoder.
In the slotted mode, to demodulate and decode the message contained within its allocated time slot, the mobile station xe2x80x9cwakes upxe2x80x9d one frame ahead of time to synchronize the Viterbi decoder within the mobile station. This early wake-up is necessary because the paging messages are continuously encoded (i.e., the base station encoder shift register is not reset in between page messages) and the decoder has no knowledge of the initial encoder state at the start of the allocated slot. Usually, the decoder requires at least a 13-bit transition to synchronize; however, since the transmitted message is interleaved over a 20 ms frame, the synchronization takes 20 ms. For most cases, there is no message transmitted to a particular mobile station, which then goes back into the sleep mode. As such, each time the mobile station does not receive a message, it spends 40 ms in the active mode, i.e., 20 ms for the wake-up period and 20 ms thereafter in its allocated slot attempting to receive a message. This represents twice as much time in the active mode as necessary (40 ms vs. 20 ms), thus significantly reducing the mobile station""s idle time capacity.
Recently, a method for improving power conservation in mobile stations operating in the idle mode in accordance with the IS-95 requirements has been proposed. The method involves at least partially flushing (i.e., partially resetting) the encoder just prior to the onset of a synchronized paging slot, by inserting at least four padding bits (e.g., four zeroes) immediately prior to the onset of the synchronized page message. For base station encoders in accordance with the IS-95 standard, an 8-bit encoder register is employed such that nine (K-1) consecutive known padding bits are necessary to reset the encoder to a known state. The method takes advantage of the fact that, due to the variability in the length of the previous message, padding bits are usually added to the end of the message anyway in accordance with the standard. The method then assumes that the initial encoder state at the onset of the allocated time slot is all zeroes (eight zeroes) since, by guaranteeing at least four zeroes, there will be more zeroes at the tail end of the previous slot most of the time. The Viterbi decoding process would then proceed based on the assumption of an all zeroes state. Consequently, the Viterbi decoder need not be turned on one frame early to synchronize, thereby conserving power. A shortcoming of this approach, however, is that the assumption of the encoder being in an all zeroes state with only four consecutive zeroes guaranteed is not valid in most cases. Indeed, simulations have shown that the message error probability with this approach is unacceptably in the range of 15 to 25%.
It is, therefore, an object of the present invention to provide an improved decoder for decoding a message transmitted in a channel in which continuous convolutionally encoded messages are transmitted without an encoder reset operation between messages.
It is another object of the present invention to improve power consumption of mobile stations during idle mode operation, while providing low message error probability.
It is a further object of the invention to advance the state of the art of wireless mobile stations.
In an illustrative embodiment of the invention, there is provided a decoding method for use in a communications system employing a communication channel in which a message is convolutionally encoded by a base station encoder and transmitted to a remote terminal during a time slot allocated to at least that remote terminal. The encoder is not completely reset immediately prior to the allocated time slot such that the encoder is in an unknown state at the onset thereof. The decoding method includes the steps of assigning a most likelihood probability for an initial encoder state to a number of predetermined encoder states; and, convolutionally decoding succeeding bits of the message based on an assumption that the initial encoder state is one of the predetermined states.
The method has particular utility when used to decode paging channel messages in a CDMA wireless telecommunications system. Preferably, a Viterbi decoder is used to perform the decoding, and the predetermined encoder states assigned a most likelihood probability are sufficient to enable the trellis of the decoder to converge within a small number of transitions, less than the minimum free distance of the code. This technique results in error-free decoding, provided that there are no consecutive bit errors in the communication channel.
The illustrative method and decoders embodying the same beneficially afford low error rate decoding of convolutionally encoded messages without the necessity of resetting the encoder on the transmit side in between messages. The invention can be employed to lower power consumption within a mobile station operating within the slotted mode, since the mobile station need not awaken substantially before the onset of its allocated time slot to synchronize its decoder.